Damage formation and annealing studies of low energy ion implants ...

Chapter 2 Atomic collision in solids
2.1 Introduction
Ion implantation is a process of injecting energetic ions into solids. Removal of
an electron from an atom leaves a positively charged ion that can be accelerated in an
electric field, and deflected in both electric and magnetic fields. The interaction of
energetic ions with atoms in solids is of fundamental importance for understanding the
behaviour of the implanted dopant ions as well as for the MEIS analysis technique. In
this chapter a review of the relevant ion / solid interactions is carried out. The review
concentrates on the atomic physics relevant to the specific energy regions associated
with low energy implantation and MEIS. It is not intended to be an exhaustive review of
ion – solid interactions. The topic is considerably more complex than the description
here. The MEIS analysis technique is discussed further in chapter 4.
Much of the understanding of the theory of particles travelling through matter
began with the work of Bohr (1, 2). As an energetic ion (or atom) moves through a
solid, it interacts and collides with lattice atoms. This interaction is due to the force of
repulsion between the two nuclei and between the two electrons clouds; and attraction
between nuclei and electrons. It is usually described in terms of an interatomic potential,
the precise form of which depends on the degree to which the nuclei penetrate the
electron screen. For both the areas of present interest, the overall scattering can be
described in terms of simple binary collisions or sequences of such collisions. However
in addition to losing energy via such collisions the incident particle loses energy directly
to the electrons in the solid and particularly for high energy light ions, the latter is the
dominant process. As a simple rule of thumb, when the energy in keV is less than the
ion mass in amu (E < M) then elastic energy loss dominates. When the energy is greater
than the ion mass (E > M), inelastic energy loss dominates (14).
In the context of the present study, for low energy implantation, the theoretical
interest is in the calculation of the depth distributions of the implanted ions and defects,
while for MEIS the dependence of the energy of the scattered projectile ions on
scattering angle and the depth at which the scattering centre is located is the primary
interest. Essentially the relevant theory is required to relate the yield of the detected
scattered ions as a function of energy at a reference scattering angle to the density and
depth distribution of the different scattering atoms. It is necessary therefore, to consider
the theory of elastic scattering processes in order to determine the energy loss per
collision as a function of the scattering angle and the masses of the colliding particles
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